If human CO2 emissions continue unabated, the earth could become a difficult place to live before the end of the century, according to a new study in PNAS.

Researchers from Purdue University and the University of New South Wales, Australia calculated the highest tolerable "wet-bulb" temperature and found that this temperature could be exceeded for the first time in human history if future climate change scenarios are to be believed.

Wet-bulb temperature is equivalent to what is felt when wet skin is exposed to moving air. It includes temperature and atmospheric humidity and is measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it.

This map shows the maximum wet-bulb temperatures reached in a climate model from a high carbon dioxide emissions future climate scenario with a global-mean temperature 12 degrees Celsius (21 degrees Fahrenheit) warmer than 2007. The white land areas exceed the wet-bulb limit at which researchers calculated humans would experience a potentially lethal level of heat stress.

(Photo Credit: Purdue University graphic/Matthew Huber)

The study did not provide new evaluations of the likelihood of future climate scenarios, but researchers used climate models to compare the peak wet-bulb temperatures to the global temperatures for various climate simulations. They found that the peak wet-bulb temperature rises approximately 1 degree Centigrade for every degree Centigrade increase in tropical mean temperature.

Researchers also calculated that humans and most mammals, which have internal body temperatures near 98.6 degrees Fahrenheit, will experience a potentially lethal level of heat stress at wet-bulb temperature above 95 degrees sustained for six hours or more.

"Although areas of the world regularly see temperatures above 100 degrees, really high wet-bulb temperatures are rare," said Matthew Huber, a Purdue professor of earth and atmospheric sciences.

"This is because the hottest areas normally have low humidity, like the 'dry heat' referred to in Arizona. When it is dry, we are able to cool our bodies through perspiration and can remain fairly comfortable. The highest wet-bulb temperatures ever recorded were in places like Saudi Arabia near the coast where winds occasionally bring extremely hot, humid ocean air over hot land leading to unbearably stifling conditions, which fortunately are short-lived today."

The challenges presented by the future climate scenarios are daunting in their scale and severity. "We found that a warming of 12 degrees Fahrenheit would cause some areas of the world to surpass the wet-bulb temperature limit, and a 21-degree warming would put half of the world's population in an uninhabitable environment," Huber said.

"When it comes to evaluating the risk of carbon emissions, such worst-case scenarios need to be taken into account. It's the difference between a game of roulette and playing Russian roulette with a pistol. Sometimes the stakes are too high, even if there is only a small chance of losing."

Humans at rest generate about 100 watts of energy from metabolic activity. Wet-bulb temperature estimates provide upper limits on the ability of people to cool themselves by sweating and otherwise dissipating this heat. In order for the heat dissipation process to work, the surrounding air must be cooler than the skin, which must be cooler than the core body temperature.

The cooler skin is then able to absorb excess heat from the core and release it into the environment. If the wet-bulb temperature is warmer than the temperature of the skin, metabolic heat cannot be released and potentially dangerous overheating can ensue depending on the magnitude and duration of the heat stress.

Comments

This, unfortunately, is quite true. Believe it or not I had learned of the predictions while I was still an undergraduate at UIC over 35 years ago from my mentor, Professor Kelvin S. Rodolfo. What is frightening to me is how accurate his predictions were at the time. He told us what would happen if the Earth's global temperature were raised by only 1°. And now we've learned that it has been raised since that time by 3°.

If anything, the problem has been understated. And what most people don't seem to understand is that we will not see the full effects of the greenhouse gases that we have already put into the atmosphere until long after we are dead. And not only haven't we even cut back of the emissions of greenhouse gases such as CO2 but during the eights years of the Bush administration, CO2 emissions from the burning of coal were increased by 37% in this country alone.

I wish that I had more encouraging things to say about this matter, but the facts simply do not support an optimistic view of the current situation and its future long term consequences. And you may find it hard to believe, but I am generally an optimist in my attitude about things, but not when the science is contrary to what I would like to believe is the case. That perhaps more than anything is what makes it so hard on a personal level to adhere to the discipline of science. Excepting the truth of what the empirical evidence tells you when it is in direct opposition of what you wish the future to be, is the most difficult and emotionally painful for any true scientist to come to terms with. But it is a tacit sacred oath to the pursuit of the truth that any true scientist has taken, regardless of any personal suffering that the revelations derived from the evidence may cause.

Fully agree with the analysis and lack of optimism. In my early adult years I was involved with environmental groups because I could perceive great problems emerging and this was independent of the AGW issue. By my late 20's I looked up and realised that the Green movement had gone slightly mad so left it and hoped that by then most would realise addressing environmental issues was not a choice but an imperative. By my mid-30's I declared my resignation from the human race because I considered it increasingly insane. It is so frustrating.

As an afterthought over the last few days I have been considering the idea that the potential success of any system, be it conceptual, technological, or cultural, is strongly predicated upon the quality of information within the system. So when politicians lie to us are they paving the way to hell?

BTW, as an added sinister twist, there is always the possibility that increasing temperatures will shrink us and perhaps dumb us down:

Eric: I take what you say on board, but I live in hope. We have somehow managed to stumble along without a nuclear holocaust. Maybe we can stumble along without a climate induced population crash. I repeat: I live in hope.

I have lived through one of the greatest fears ever: the threat of global thermonuclear war. During the 1950s the theme of the destruction of humanity was picked up in many sci-fi books and films. The fear of destruction was real and was ever present, and fictionalisation was cathartic. Unless they have read Bobby Kennedy's memoirs and other such documentation, people don't realise just how close the world came to WW3 over the Cuban missile crisis. There have also been various 'hiccups' in military equipment. For example, when the DEW line was first put into operation it detected what appeared to be a huge Soviet attack. It was the moon rising. The Soviets had their own version of that scenario in 1983. That time the world was only minutes from WW3, saved by a soldier who had the good sense to disobey standing orders and who refrained from pressing the button. So that's the biggest ever tip of the hat from me to Станислав Евграфович Петров - Stanislav Yevgrafovich Petrov.

The madness of nuclear standoff was visible to everyone. With the possible exception of Senator Joseph McCarthy. There were plenty of movies, documentaries and scientific analyses to show what would happen if a nuke dropped on your city. No science degree needed.

M.A.D. - Mutual Assured Destruction. There is a brilliant analysis of M.A.D., as portrayed in the movie Dr. Strangelove, by Dan Lindley. It deserves a wider readership.

Business as usual in the face of global warming is as mad as MAD. The risk is a s great if we get it wrong. This time, however, the risk is non-intuitive and we have more Joe McCarthys than you can shake a stick at. Maybe there is a psychological barrier to horrendous truth. Maybe some people would rather pick holes in someone else's math than go look at the Arctic melting. Maybe it's more about human psychology than the ethics of scientists.

Climate scientists cleared of any wrongdoing continue to be accused by the deniersphere of exaggerating global warming induced climate change. Thanks to propagandists, most people link CO2 with Mann's hockey stick graph. All the other similar graphs which validate the data are unheard of. The proverbial 'man on the Clapham omnibus' has never heard of Mauna Kea or the Keeling curve. We have known that fossil fuel burning has the potential to warm the planet since about 1860. Global warming has been accepted as a fact by scientists since the 1970s. Since the 1970s a concerted propaganda campaign by the fossil fuels industry, mining industries and religious fundamentalists has prevented all attempts at addressing the problem.

There are push-pull mechanisms at work on climate. At the moment they are nearly in balance. But if they ever get to push and pull in harmony, global temperatures will climb, and sea levels will rise, at such a pace as to alarm even the most ardent critic of the IPCC. At that time, I would not be a global warming denying propagandist for all the gold in the Bank of England.

Patrick, you presume too much, For example, like I have never seen the movie Dr. Strangelove with Peter Sellers or that I didn't live growing up through the Cold War and Mutually Assured Destruction (MAD). And I've got news flash for you: most people don't even know this, but we are in a 21st century Cold War with the Chinese. Let me explain. For obvious reasons our power grid is a part of the Internet just as the Chinese power grid is a part of their Internet. The part that most people don't know is that China hacked into our power grid many years ago just as we have hacked into theirs. Each of us has implanted very sophisticated Trojan horse programs with deadly payloads at critical junctions in each of our respective grids. In a word, either of us could send the other nation back to a pre-industrial existence permanently. We are in effect in a stalemate, id est, MAD.

But that's not what we're talking about here, Patrick. We're talking about natural processes that have already been set into motion, the consequences of which none of us knows. This isn't Man screwing with Man! This is Man screwing with Nature! And Man doesn't have a clue!

The damage has already been done and there's not a damn thing we can do to reverse it! The question isn't whether we should stop immediately what we're doing. That's a given. The point is have what we have already done made it too late for things to in time reverse themselves before our extinction as a species?

Even the best scientists in the world don't know the answer to that question. And I know, because I am one of the best scientists in the world!

And to make matters worse we now have a stratovolcano pouring only God knows how many metric tons of CO2 not to mention other greenhouse gases into our atmosphere each day. And only God knows when this volcano is going to stop!

I've got to be honest with you, Patrick. I'm beginning to wonder if the end-of-the-worlders aren't right! The difference is, if they are right, it is for all of the wrong reasons. They do not understand the physical processes at work here. I, on the other hand, do! But this is not the first time that this has happened in the history of the Earth. Nor will it be the last. It's just the first time that we've had the technology to contribute to and accelerate the process, before we even had a chance to figure it all out. And that truly is a tragedy that even Shakespeare couldn't have possibly imagined in his time.

But even the most ridiculous assertions in the salad days of global warming hysteria (2006) did not speculate any way for there to be a 12 degree jump. That's not a reason to pile on pollution but wet bulb temperature won't be the thing that kills us.

Hank is right. I sometimes forget the volume of lava and quantity of greenhouse gases that the mid-Atlantic ridge put out over a very long period of time while Pangea was breaking apart. I'm just being silly. In the worst case scenario, even if this thing turns into a supervolcano, which it could, there's no way that it could pour enough carbon dioxide and other greenhouse gases into the atmosphere that could even in the long term come close to raising the global temperature by even 2°C--maybe 1°C at most over a period of time.

I guess, I am more worried about the short term affects that this volcano could have on the Icelandic people. I've seen a lot of powerful eruptions in my time, but I have never seen one as powerful as this one! I don't think anyone saw this one coming.

Even I sometimes get carried. ;-) When it comes to stratovolcanoes, especially those that have the potential of becoming supervolcanoes, even I can't always maintain my emotional detachment and remain rational about these things.

Yep, this sort of speculation simulation is fun for big media because it gets people worked up but we don't want to distract people too much from the actual pressing issues we have today. Doomsday speculation may make people just give up.

What's got me worried, Hank is this curve ball that's been thrown at us. This stratovolcano in Iceland can make all the difference in the world. Normally, I don't worry that much about the eruptions of stratovolcanoes. But this one is sitting practically on top of the mid-Atlantic ridge. And it was the formation of the mid-Atlantic ridge which caused the galloping greenhouse effect during the Jurassic. Quite frankly, I don't know how much greenhouse gases is going to come out of this thing before it's finished. But it could be enough to tip the balance.

The amount of greenhouse gases that we put into the atmosphere in a year is less than what a stratovolcano like this can put out in a month! And if this sucker follows its historical pattern, it could keep erupting for over a year. And grounded air-flights will be the least of our problems if that happens.

I've just got a very bad feeling about this. This is one of the very rare occasions when I hope I am wrong. Nothing would make me happier than to be wrong about this.